1. Field of the Invention
A permanent magnet electric machine of the axial air gap type that can adjust the axial gap field strength to increase the speed of the machine without increasing power by radially shifting the magnets under the influence of centrifugal force acting against spring resistance.
2. Description of Related Art
Back-emf in the machine is directly proportional to the gap flux density, the length of the conductors and the speed of rotation. As the speed increases, the back-emf continuously rises. At a certain speed, the back-emf will equal the applied voltage. This specific speed is called base speed. The motor cannot be rotated beyond the base speed because the back-emf exceeds applied voltage (which is assumed to be fixed and unalterable). In other words, if there is no mechanism to reduce back-emf, the base speed defines a theoretical limit beyond which the motor cannot rotate.
A conventional permanent magnet motor can provide high torques up to base speed, but cannot usually operate beyond it. Achieving maximum speeds that are a factor of three or five times higher than the base speed is difficult and has presented serious technological problems. Several methods have been developed to operate a permanent magnet motor beyond the base speed. These methods can be broadly grouped into electrical means, mechanical means and centrifugal means. The electrical means manipulate voltage or currents applied to the coils, the mechanical means employ a stator adjusted by an external actuator, while the centrifugal means employ movable rotor magnets that are moved by centrifugal action to alter the gap flux.
Several electrical means of enhancing the speed of permanent magnet machines have been devised. These include increasing the number of phases, deactivating some phase coils as speed increases, shaping the current, weakening the gap flux, boosting the applied voltage, changing the phase between the two, or switching from a wye-mode to a delta mode. U.S. Pat. No. 5,677,605 issued Oct. 14, 1997 to Cambier et al and U.S. Pat. No. 5,739,664 issued Apr. 14, 1998 to Deng et al and U.S. Pat. No. 4,546,293 issued Oct. 8, 1985 to Peterson et al, illustrate some of these approaches. The flux weakening technique uses large currents to suppress the field generated by the permanent magnet, thereby reducing the gap field. Large currents, however, increase temperature and potentially damage the magnets. This technique is applied mostly to sine-wave driven motors where mutual inductance between two phase coils is relatively large and non-negligible. It cannot be readily applied to square-wave driven systems, especially when the mutual inductance between the phase windings is negligible, even though attempts have been made to use it for surface-permanent-magnet machines. The voltage boost technique requires additional dc--dc converters to enhance the voltage applied to the motor windings. This, however, increases the cost of the motor drive and increases its weight while its efficiency of operation is reduced.
Mechanical means of enhancing the speed employ a movable stator to bring it closer to rotor magnets, thereby reducing or increasing the axial air gap. U.S. Pat. No. 2,719,331 issued Oct. 4, 1955 to E. Harris and U.S. Pat. No. 2,784,332 issued Mar. 5, 1957 to W. Kober, and U.S. Pat. No. 2,824,275 issued Feb. 18, 1958 to W. Kober, and U.S. Pat. No. 2,892,144 issued Jun. 23, 1959 to W. Kober, disclose means for flexibly mounting the stator such that the axial air gap can be increased or decreased as the load changes. U.S. Pat. No. 3,250,976 issued May 10, 1966 to E. McEntire and U.S. Pat. No. 5,627,419 issued May 6, 1997 to R. Miller, describe a radial gap machine in which the stator can be moved axially. Such methods require complex external energy sources with the overall efficiency of the motors reduced by the energy consumed by these actuating devices.
Centrifugal means to enhance motor speed do not require external devices or energy sources and hence they are self-actuating. They are preferable since they do not reduce the overall efficiency of the machines. The principal used is that rotary permanent magnets tend to fly radially outward under the action of centrifugal force. They have been proposed only for radial gap machines. U.S. Pat. No. 1,153,076 issued Sep 7, 1915 to J. Heinze discloses a hinged magnet and lever mechanism to move the rotating magnets radially outward thereby opening or increasing the radial air gap. U.S. Pat. No. 5,053,659 issued Oct. 1, 1991 to D. Parker also discloses a device for moving permanent magnets using centrifugal force.
These prior art centrifugal devices that are applied to radial gap machines suffer from several problems, namely opposing attraction forces, frictional losses and excessive flux leakage. The opposing attraction force problem stems from the fact that the centrifugal force must be large enough to overcome large attraction forces that exist between the rotor magnets and the stator in radial gap motors. As a result, a very small portion of the centrifugal force is available to increase motor speed. In addition, in radial gap motors the act of moving the magnets introduces significant flux leakage reducing the effectiveness of the magnets.